Kickresume has beautiful templates, with good colors, font and format customization power. It is very simple to use and bunch of different other feature like importing in doc or pdf format and edit it. they also have personal websites creation in pro plan.

Since you asked for a tool, instead of a Cover Letter Template, i am recommending this. It has a simple wizard like interface, where it asks for each part of the cover letter content in an incremental fashion. Although i do not like the styling of the end result. But it's good if the focus is more on the content.

This tool has a bunch of great cover letter templates that you can get inspiration from. Unlike a lot of the cover letter tools out there, the content on these templates are actually good (i.e. free from grammatical errors, structured correctly, etc)

The cover letter is a tool to help introduce yourself in a memorable, personal way during a job application. You should always submit a cover letter with your CV. A well written cover letter goes over information on your resume and add more information for the recruiter.